Radiation Pressure | Physics with Professor Matt Anderson | M25-17

Summary

The video script explores the concept of radiation pressure, explaining how sunlight not only heats but also exerts a small force on objects. It delves into the physics of momentum transfer from light waves to materials, whether absorbed or reflected. The script uses the example of a solar sail in space, demonstrating how the pressure from sunlight can generate a small but continuous force, sufficient to propel a spacecraft over time without the need for fuel. This innovative application of solar radiation pressure highlights the potential for long-term, sustainable space travel.

Takeaways

• 🌞 Sunlight exerts a small amount of pressure on objects it hits, including people on Earth.
• 🔥 When sunlight is absorbed, it not only transfers energy but also imparts a small force, causing a momentum transfer.
• 📐 The momentum transfer from a wave to an absorber is calculated as the absorbed energy divided by the speed of light (Δp = ΔU / c).
• 🔄 For a reflector, the momentum transfer is twice that of an absorber, similar to the difference between a super ball and a sticky ball hitting a wall.
• 🔽 Force is related to the change in momentum over time (F = Δp / Δt), and from this, radiation pressure can be derived.
• 🌌 The radiation pressure on an absorber is the average intensity of sunlight divided by the speed of light (P = S̄ / c).
• 🛸 Solar sails, like those used in spacecraft, utilize the pressure exerted by sunlight to propel themselves through space.
• 📏 The force exerted on a solar sail can be calculated using the formula: force = pressure × area.
• 🔆 The average intensity of sunlight at Earth's location is approximately 1400 watts per square meter.
• 🚀 A solar sail with an area of 50m x 50m would experience a force of 0.023 Newtons, which is quite small.
• 🌠 Despite the small force, the continuous acceleration due to sunlight can be significant over time in the vacuum of space.

Q & A

What effect does sunlight have on a person standing on Earth?

Sunlight heats up a person when it is absorbed, making them feel warm or hot.

Does sunlight also exert a pushing force on a person standing in it?

Yes, sunlight exerts a small pushing force due to radiation pressure, although it is generally not noticeable.

What is the concept of radiation pressure?

Radiation pressure is the force exerted by electromagnetic waves, such as sunlight, when they transfer energy and momentum to an object upon hitting it.

How is the momentum transfer from a wave to an absorber calculated?

The momentum transfer (delta p) to an absorber is calculated as the energy absorbed (delta u) divided by the speed of light (c).

What is the difference in momentum transfer between an absorber and a reflector?

For a reflector, the momentum transfer is twice that of an absorber, similar to how a super ball transfers twice its momentum to a wall compared to a sticky wad of clay.

How is force related to momentum?

Force (F) is related to momentum (delta p) and time (delta t), with the formula F = delta p / delta t.

What is the formula for calculating pressure due to radiation on an absorber?

The pressure (P) on an absorber due to radiation is given by the formula P = average intensity (s bar) / speed of light (c).

How does the pressure formula change for a reflector?

For a reflector, the pressure formula is P = 2 * (s bar / c), indicating a doubling of the pressure compared to an absorber.

What is a solar sail and how does it work?

A solar sail is a large, reflective surface attached to a spacecraft that uses the pressure exerted by sunlight to propel the spacecraft through space.

What is the average intensity of sunlight at Earth's location and how is it related to the force on a solar sail?

The average intensity of sunlight at Earth's location is about 1400 watts per square meter. This intensity, when multiplied by the area of the solar sail and divided by the speed of light, gives the force exerted on the sail.

Why would a small force from a solar sail be useful for space travel?

Although the force from a solar sail is small, it is continuous and there is little resistance in space, allowing for a slow but constant acceleration that can eventually lead to high velocities.